The present invention relates to a method for determining the amount of charge which can still be drawn from an energy storage battery and to an energy storage battery having measurement means and processor-controlled evaluation means for carrying out the method.
Wear occurs as the result of the use of rechargeable energy storage batteries, in particular while they are being discharged and charged. In addition, other operating conditions also exist which accelerate the wear of electrochemical energy stores. For example, in the case of a lead-acid rechargeable battery, these include the total operating time, that is to say the total time which has passed since they were brought into use, including those periods in which the rechargeable battery was not subject to any electrical load. This wear is further exacerbated by increased temperatures. Increased temperatures may, however, not only accelerate the wear during periods when no electrical load is applied, but may also increase the wear caused by cyclic discharging and charging.
For an energy storage battery, the wear becomes evident inter alia from a decrease in the storage capacity for electrical charge, so that the amount of charge which can be drawn cannot simply be determined on the basis of the full state of charge.
The amount of charge QRest which can be drawn from the energy storage battery is in this case that amount of charge which can still be drawn in nominal conditions starting from the present state of the energy storage battery. When the energy storage battery is new, the sum of the amount of charge which can be drawn and the amount of charge which is being discharged is the storage capacity when new. The storage capacity when new is the actual storage capacity of a new, unused energy store.
The amount of charge which has been discharged is that amount of charge which must be drawn in nominal conditions in order to reach the present state of charge, starting from an energy storage battery in the fully charged state.
The amount of charge, expressed in ampere hours (Ah), which can be drawn in nominal conditions from a correctly fully charged energy store is defined as the present storage capacity. This parameter generally varies with a falling trend as the usage duration increases.
The nominal value of the storage capacity as specified by the manufacturer of the energy storage battery is defined as the nominal storage capacity. The reduction in the storage capacity is understood to be the difference between the present storage capacity and the storage capacity when new.
U.S. Pat. No. 5,721,688 discloses an apparatus and a method for measuring electrical characteristic variables of a power supply system using a voltage measurement apparatus, a controllable current source or current sink, and a microcomputer. The microcomputer is used to control the current source or current sink such that at least one specific current profile is applied to the power supply system. The voltage measurement device is then used to measure the voltage response to the current profile and to determine an electrical characteristic variable from this, with one of the characteristic variables being at least the internal resistance of the power supply system. The internal resistance is determined from the voltage difference between the voltage drop across the resistor in test conditions and the voltage drop when a test current is superimposed on the current that flows in the operating state.
U.S. Pat. No. 5,572,136 discloses an electronic apparatus for testing energy storage batteries, by means of which a relatively small current which varies with time is applied to an energy storage battery, and the time-dependent voltage response of the energy storage battery is observed. A microprocessor is used to determine the conductivity of the energy storage battery from the voltage response. Furthermore, the observed voltage response level can be compared with a reference value in order to derive an assessment of the quality of the energy storage battery. In this case, the profile of the voltage response is recorded and is evaluated. This is relatively complex and leads to inaccuracies in the derivation of a comparable characteristic variable.
DE 93 21 638 U1 discloses an electronic battery tester for testing an electrochemical energy storage battery, which has a dynamic parameter (conductance or resistance). The battery tester has means for measuring this dynamic parameter. Furthermore, the no-load voltage is measured, and the measured dynamic parameter value is corrected for the state of charge by matching it to the no-load voltage.
U.S. Pat. No. 5,680,050 discloses a method for identification of the battery state, in which a correction value is determined on the basis of an average discharge current for a time period which is longer than that time period which results in the discharge polarization assuming a steady state. The capacity of the energy storage battery in the fully charged state is multiplied by the correction value, and the energy consumption of the battery is subtracted from the available discharge capacity in order to determine the available reserve capacity.
The methods and apparatuses described above are used for determining present electrical characteristic variables of an energy storage battery in terms of the conductance and/or internal resistance. The methods also in some cases make it possible to determine the discharge capacities which can be drawn from an energy storage battery as a function of the charging current. However, the capacity of the fully charged energy storage battery must be known in order to do this.
Furthermore, DE 691 31 276 T2 discloses an electronic tester for assessing the percentage energy capacity of an energy storage battery or of a battery cell. In this case, the dynamic conductance is determined and is compared with a reference conductance, which corresponds to the conductance of a cell or energy storage battery with a capacity of 100%. However, this method does not allow the total capacity of a fully charged energy storage battery to be established, and does not make it possible to determine whether any capacity change has occurred due to aging of the energy storage battery or due to discharging during operation. Furthermore, no information about the wear state of the energy storage battery can be obtained from the time response of the measured values over a lengthy time period. The dynamic conductance is useful only to a limited extent as a measure for battery wear.
There is thus a need to provide an improved method for determining the amount of charge QRest which can still be drawn from an energy storage battery when this energy storage battery is no longer new. There is also a need to provide an energy storage battery having measurement means and processor-controlled evaluation means for carrying out the method.